Equalization circuit

ABSTRACT

A frequency equalization circuit includes an active filter circuit. The filter circuit differentiates the input signal. A plurality of adjustable resistances are connected to the differentiator corresponding to a plurality of operating or frequency ranges. A switch selects one resistance in accordance with one selected operating range to connect the filter circuit to an amplifier.

BACKGROUND OF THE INVENTION

There are a number of applications where a high frequency equalizationcircuit is needed to provide a signal rising or falling in amplitudewith frequency over a plurality of possibly overlapping operating rangeseach operating range covering a range of frequencies and adjustablewithin each range. One significant area of application is in signalrecorders and especially audio recorders. Other applications exist suchas plural range tone controls.

Due to the signal transfer characteristics of present audio recorders,and especially the characteristics of recording tape, the high frequencyrange of the audio signal becomes attenuated by the recording andreproduction process. This attenuation occurs to varying degreesdependent upon a number of variables including relative transducer torecord medium speed and the frequency characteristics of the tape.Consequently, an adjustable high frequency amplitude equalizationcircuit is commonly employed in the audio record circuitry of therecorder to compensate for the attenuation. The audio equalizationcircuitry may be designed into the audio preamplifier.

In the past, passive RC filters utilizing adjustable capacitors havebeen used with limited success as adjustable amplitude equalizers. Forexample, many multiple speed audio tape recorders have tape speedcapabilities ranging from 33/4 inches per second to 30 ips. Recordershaving a wide range of selectable operating speeds have required foreach speed a separate filter with a variable capacitor. While suchpassive equalizers effect equalization, they do so at the expense ofappreciable overall signal attenuation and concomitant loss of signalpower and quality. Because the filters require a large number ofelements and a variable capacitor for each tape speed, the passiveequalizers are expensive.

It is preferable to utilize an active filter to minimize signalattenuation. The use, however, of a separate active filter for each tapespeed in a multiple speed machine can increase expenses beyond the costof using separate passive filters.

Many filter circuits, both passive and active, can introduce asignificant amount of noise into the signal. Many introduce noise whenthe equalization provided by the circuit is adjusted to zero. The samelevel of noise may be introduced regardless of the amount ofequalization provided. Active filters, moreover, may introduce noiseinto the system inversely to the level of signal amplitude enhancementor equalization introduced. This later case is especially undesirablebecause the signal to noise ratio becomes high with small amounts ofequalization.

SUMMARY OF THE INVENTION

Accordingly, the present invention constitutes a high frequencyamplitude equalization circuit, preferably for the record circuitry of asignal recorder, that is adjustable to provide a desired signalfrequency response over a wide range of relative transducer to recordmedium speeds. The signal to be equalized is passed through two paths,one of which includes an active filter circuit. The active filtercircuit is responsive to the signal to vary with frequency the amplitudeof any of its frequency components lying in a predetermined frequencyrange. In one preferred embodiment, the active filter circuit isconstructed to vary the amplitude of the signal's frequency componentsin a high frequency range. The signal from the active filter circuit iscombined with the signal passed through the other of the two paths forfurther utilization. The active filter circuit comprises adifferentiating circuit, an amplifier and a plurality of adjustableresistances selectable one for each machine operating speed. Preferably,the differentiating circuit is an RC circuit connected to the input ofthe amplifier. The output of the amplifier, which is varying withfrequency, is adjusted by one of the plurality of adjustable resistancesselected according to desired speed thus providing equalization over oneof a plurality of operating or frequency ranges. The RC differentiatorand amplifier are utilized for all operating speeds. Additionaladjustable resistances only are needed for additional operating speeds.In the preferred embodiment, the filter circuit is placed in parallelwith the input resistance of an operational amplifier in the audiopreamplifier circuit. Thus, the input signal follows two paths. In onepath, the signal is fed directly to the amplifier through the inputresistor and in the other path, the signal is fed through the filtercircuit to the amplifier. Preferably, the adjustable resistances areplaced in parallel between the output of the filter amplifier andground. This arrangement causes a minimum of noise to be introduced intothe signal. When no high frequency equalization or boost is introducedinto the circuit, i.e., when the tap of the adjustable resistanceselected is turned to ground, no noise is introduced. Other additionalpre-emphasis circuits may be conveniently added to affect a lowfrequency amplitude boost or to provide other compensation, if desired,by using the feedback path from the output of the summing amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of an audio preamplifier utilizingan embodiment of the equalization circuit of the present invention.

FIG. 2 is a graph representing the varying response curves availableutilizing the present invention.

FIG. 3 is a schematic circuit diagram of an alternative embodiment ofthe equalization circuit of the present invention utilizing additionalfilter pre-emphasis circuits.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, 1, terminal 1 is the input to the audio preamplifier shown.Terminal 2 is the output. The main signal path of the preamplifiercomprises input resistor 4, amplifier 3, and feedback resistor 5connected between the amplifier input terminal 6 and the output terminal2. The gain of the amplifier 3 is, with little error: ##EQU1## Where A =the closed loop gain of the operational amplifier.

R₅ = the resistance of feedback resistor 5

R₄ = the resistance of input resistor 4

Connected in parallel with input resistor 4 is an active filter circuit10, which is designed to perform a high frequency signal amplitudepre-emphasis. Any signal applied to input terminal 1 will thus passthrough input resistor 4 and the filter circuit 10. The resultantsignals will be summed at terminal 6 and then amplified by amplifier 3.The signal passing through the filter circuit is emphasized andthereafter summed with the signal passing through resistor 4 andamplified by amplifier 3 to form the desired equalized signal.

The active filter circuit 10 comprises an amplifier 11 with a feedbackresistor 12. Connected between terminal 1 and the input 24 of theamplifier 11 is a series RC network comprising resistor 13 and capacitor14. A plurality of adjustable resistors 15, 16 are connected between theoutput 25 of amplifier 12 and ground. There is one adjustable resistorfor each operating speed. Connected respectively to the adjustable tapsof adjustable resistors 15, 16 are connecting resistors 18, 19.

A switch 20 selectively connects the amplifier's input terminal 6 to oneof the connecting resistors 18, 19. One terminal 21, 22, 23 of switch 20is selected according to the selected operating speed.

In operation, an audio signal is coupled to terminal 1. The signalpasses through the path formed by resistor 4 and also through the pathformed by resistor 13, capacitor 14 and amplifier 11 where it isdifferentiated and amplified. The differentiation of the signal causessignal amplitude in this embodiment to rise with frequency above apredetermined frequency as described below. The selection of thecomponent values of the active filter circuit determines the frequencyrange of the resultant high frequency pre-emphasis. In the preferredembodiment the output signal of amplifier 11 rises at 6 db/octave withincreasing frequency. The limit to this rise is set by R₁₃ C where R₁₃is the value of resistor 13 and C is the value of capacitor 14.

One terminal, for example 21, of switch 20, is selected according to achosen operating speed to connect adjustable resistor 15 to terminal 6by way of resistor 18. Accordingly, a portion of the differentiatedsignal dependent on the position of the tap of the adjustable resistoris summed with the undifferentiated signal, which has passed throughresistor 4 to terminal 6. The resultant signal is amplified byoperational amplifier 3 and available as an equalized output at terminal2.

The frequency above which the signal is pre-emphasized by the activefilter circuit 10 is determined by: ##EQU2## where R₄ = the resistanceof resistor 4

R₁₂ = the resistance of resistor 12

R₁₃ = the resistance of resistor 13

R₁₈ = the resistance of connecting resistor 18

C = capacitance of capacitor 14

K = the fraction of the adjustable resistor 15 between the tap andground

f₁ = frequency.

The pre-emphasis frequency, then, is dependent on the positioning of thetap on the adjustable resistor 15. The lowest pre-emphasized frequencyis determined, when K = 1, by the values of R₄, R₁₂, R₁₈ and C. When thetap is set to ground, K = 0 and there is no pre-emphasis.

The upper frequency limit of signal pre-emphasis is determined by##EQU3##

It can be readily seen that where different amplitude equalization isrequired by different tape speeds, only the adjustable resistor 15 andthe connecting resistor 18 need be substituted with new values such asresistors 16 and 19. Resistors 12 and 13, capacitor 14 and amplifier 11are common to all speeds.

When there is zero boost or equalization, the tap of the adjustableresistor is set to ground and no output signal from the active filtercircuit is transferred to the main circuit path. Consequently, there isno noise contribution from the pre-emphasis circuit filter. When boostis provided, output noise remains substantially constant until K becomesclose to unity in which case some additional noise is contributed byamplifier 11 at high frequencies only. Thus a minimum amount of noise isintroduced into the circuit.

FIG. 2 shows a family of curves depicting the range of high frequencyequalization curves obtainable at a given tape speed as resistor 15, forexample, is adjusted. As resistor 15 is adjusted, frequency f₁ will varyas shown by the arrow. Frequency f₁ will vary from a minimum amount whenK=1 to higher amounts when K is reduced. The selection of anotheradjustable resistor 16 is necessary to provide a different range overwhich f₁ must vary due to different signal equalization required bymachine characteristics which change with operating speed. Along theordinate is plotted gain G, and along the abscissa is plotted frequencyf. The gain of amplifier 3 is shown at 30 and equals R₅ /R₄. The upperlimit of boost 31 is given by the equation for f₂. The family of curves32, 33, 34 represents the range of equalization that is obtained byvarying K. The lowest possible corner frequency, when K=1, is shown at35. As the corner frequency increases, the amount of signal amplitudeboost decrease until none is present in the equalization.

Accordingly, the output of amplifier 3 consists of two components, onewhose amplitude is constant with frequency and one of which iscontributed by the pre-emphasis circuit and whose amplitude rises withfrequency over a selected range of frequencies.

Additional compensating networks may be added for adjustable lowfrequency pre-emphasis or to provide other compensation. The additionalnetworks are preferably switched using the same switch as utilized forhigh frequency pre-emphasis. As shown by way of example only in FIG. 3,additional networks 40, one or more for each operating speed, may beconnected between the switch terminals 21, 22, 23 and the circuit output2. The particular low frequency pre-emphasis circuit shown consists of acapacitor 41 in series with resistor 42. In parallel with resistor 42 isanother series RC circuit comprising capacitor 43 and resistor 44. Onesimilar circuit could be provided for each range of operation. Thecircuit, of course, need not take the form specifically described, butmay be of any suitable configuration to provide the desiredequalization.

The invention provides a high frequency equalization circuit in an audiopre-amplifier. The active filter circuit includes a series RC circuitconnected to the input of an amplifier for differentiating the inputsignal to the preamplifier. The output of the filter amplifier isconnected to ground through a plurality of variable resistancesconnected in parallel. Each variable resistance provides a means to varythe amount of high frequency boost utilized at one of a plurality oftape speeds of an audio tape recorder. Accordingly, one RCdifferentiator and amplifier are utilized at all operating speedsrequiring a minimum of component change for different speeds. Due to thedescribed circuit configuration, there is a very low noise contributionto the pre-amplifier from the active filter circuit except at close tomaximum boost or pre-emphasis when the noise contribution is at highfrequencies only. Since the variable resistances are grounded at oneend, there is no noise contribution at zero boost when the taps on theresistances are set to ground. The input signal is passed both throughthe filter and through a resistor directly into the pre-amplifier wherethe sum of the two components is amplified. An alternative embodiment ofthe invention includes additional pre-emphasis circuits connected to beselectively switched with the adjustable resistances to provide lowfrequency or other desired special equalization.

Although the invention has been described herein with reference to twoembodiments, it is to be understood that various modifications may bemade thereto within the spirit and scope of the invention. Thus it isnot intended to limit the invention except as defined in the followingclaims:

What is claimed is:
 1. An equalization circuit comprising;an input;amplifying means connected to said input; filter circuit means having anoutput connected to said input for providing a signal varying inamplitude with frequency at its output; a plurality of adjustableresistances corresponding to a plurality of operating ranges connectedto the output of said filter circuit means for adjusting the signalamplitude provided by the output; and, switch means for selectivelyconnecting one of said plurality of adjustable resistances to saidamplifying means according to one selected operating range.
 2. Thecircuit of claim 1 wherein:said filter circuit means provides a signalwhich rises with frequency.
 3. The circuit of claim 1 wherein:saidfilter circuit means comprises an RC series circuit connect to saidinput.
 4. The circuit of claim 1 wherein:said filter circuit meanscomprises active filter means.
 5. The circuit of claim 1 wherein:saidfilter circuit means comprises differentiating means.
 6. An equalizationcircuit comprising:an input terminal; a first amplifier having an inputand an output; an input resistance connected between said input terminaland the input of said first amplifier; and, a filter circuit connectedin parallel with said input resistance including:circuit means having anoutput to filter a signal received at said input terminal; a pluralityof adjustable resistances connected to the output of said filter circuitmeans, each of said resistances selected according to a different one ofa plurality of operating ranges for adjusting the amplitude of thesignal provided by said filter circuit means; and, switch meansconnected between said plurality of adjustable resistances and the inputof said first amplifier for selectively connecting said adjustableresistances to said first amplifier.
 7. A circuit of claim 6wherein:said filter circuit means provides a signal varying withfrequency.
 8. The circuit of claim 6 wherein:said filter circuit meansprovides a signal rising with frequency.
 9. The circuit of claim 8wherein:said filter circuit means includes an RC circuit connected tosaid input terminal and a second amplifier connected to said RC circuit,the output of said second amplifier being connected to said plurality ofadjustable resistances.
 10. The circuit of claim 9 wherein:said switchmeans comprises a plurality of contacts connected respectively to saidplurality of variable resistances; and further including: a plurality offrequency pre-emphasis means connected respectively between saidplurality of contacts and the output of said first amplifier.
 11. Thecircuit of claim 6 wherein:said filter circuit means comprises an RCseries circuit connected to said input terminal.
 12. The circuit ofclaim 6 wherein:said filter circuit means comprises active filter means.13. The circuit of claim 6 wherein:said filter circuit means comprisesdifferentiating means.